From 1e2e28d3e442ba3c95a5c2b9e5d920c72f6cee43 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 14 Aug 2019 13:11:55 +0200
Subject: Add mkanylink() filesystem function

---
 libbutl/filesystem.cxx | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++
 libbutl/filesystem.mxx | 21 +++++++++++++++++--
 2 files changed, 75 insertions(+), 2 deletions(-)

diff --git a/libbutl/filesystem.cxx b/libbutl/filesystem.cxx
index df738ac..b310dd8 100644
--- a/libbutl/filesystem.cxx
+++ b/libbutl/filesystem.cxx
@@ -671,6 +671,62 @@ namespace butl
   }
 #endif
 
+  entry_type
+  mkanylink (const path& target, const path& link, bool copy, bool rel)
+  {
+    using error = pair<entry_type, system_error>;
+
+    try
+    {
+      mksymlink (rel ? target.relative (link.directory ()) : target, link);
+      return entry_type::symlink;
+    }
+    catch (system_error& e)
+    {
+      // Note that we are not guaranteed (here and below) that the
+      // system_error exception is of the generic category.
+      //
+      if (e.code ().category () == generic_category ())
+      {
+        int c (e.code ().value ());
+        if (c == ENOSYS || // Not implemented.
+            c == EPERM)    // Not supported by the filesystem(s).
+        {
+          try
+          {
+            mkhardlink (target, link);
+            return entry_type::other;
+          }
+          catch (system_error& e)
+          {
+            if (copy && e.code ().category () == generic_category ())
+            {
+              int c (e.code ().value ());
+              if (c == ENOSYS || // Not implemented.
+                  c == EPERM  || // Not supported by the filesystem(s).
+                  c == EXDEV)    // On different filesystems.
+              {
+                try
+                {
+                  cpfile (target, link);
+                  return entry_type::regular;
+                }
+                catch (system_error& e)
+                {
+                  throw error (entry_type::regular, move (e));
+                }
+              }
+            }
+
+            throw error (entry_type::other, move (e));
+          }
+        }
+      }
+
+      throw error (entry_type::symlink, move (e));
+    }
+  }
+
   // For I/O operations cpfile() can throw ios_base::failure exception that is
   // not derived from system_error for old versions of g++ (as of 4.9). From
   // the other hand cpfile() must throw system_error only. Let's catch
diff --git a/libbutl/filesystem.mxx b/libbutl/filesystem.mxx
index 2e3a0d9..e028975 100644
--- a/libbutl/filesystem.mxx
+++ b/libbutl/filesystem.mxx
@@ -289,8 +289,8 @@ LIBBUTL_MODEXPORT namespace butl
   // Create a hard link to a file (default) or directory (third argument is
   // true). Throw std::system_error on failures.
   //
-  // Note that on Linix, FreeBSD, Windows and some other platforms the target
-  // can not be a directory.
+  // Note that on Linux, FreeBSD, Windows and some other platforms the target
+  // cannot be a directory.
   //
   LIBBUTL_SYMEXPORT void
   mkhardlink (const path& target, const path& link, bool dir = false);
@@ -303,6 +303,23 @@ LIBBUTL_MODEXPORT namespace butl
     mkhardlink (target, link, true /* dir */);
   }
 
+  // Make a symlink, hardlink, or, if `copy` is true, a copy of a file (note:
+  // no directories, only files), whichever is possible in that order. If
+  // `relative` is true, then make the symlink target relative to the link
+  // directory (note: it is the caller's responsibility to make sure this is
+  // possible).
+  //
+  // On success, return the type of entry created: `regular` for copy,
+  // `symlink` for symlink, and `other` for hardlink. On failure, throw a
+  // `pair<entry_type, system_error>` with the first half indicating the part
+  // of the logic that caused the error.
+  //
+  LIBBUTL_SYMEXPORT entry_type
+  mkanylink (const path& target,
+             const path& link,
+             bool copy,
+             bool relative = false);
+
   // File copy flags.
   //
   enum class cpflags: std::uint16_t
-- 
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